If an image doesn't have alt text, we could try to see if it has a content description in the image metadata:

XMP metadata field: Iptc4xmpExt:AOContentDescription

This is supported on PNG and JPEG images, and in theory WEBP too. The work involved would be to extract the metadata from each codec and pass it back from the ImageLoader to Blink, and then write some tests using image files that have the metadata.